Tengu commented on my first human sexuality post and pointed me in the direction of very interesting study asking much the same question I did. Why do human females have perpetually large breasts? The study is illuminating, but leaves a few questions unanswered, though I encourage you all to read it. First of all, the authors argue that large breasts got women in hunter/gatherer societies through tough times--they basically acted like camel humps (fat stores). Well, here's my question: why the breasts? They offer tenuous evidence that breast fat somehow converts to milk, but I'd like to see more on that. The human body stores fat in all sorts of places: thights, butt, belly, and breasts. But why would natural selection drive fat stores for women only to the chest?*
Second, it large breasts were so important for early societies, why aren't they strongly written into modern human genomes? The fat-store theory is great, but it implies a pretty strong evolutionary bottleneck--you did not survive the dry season without big breasts. So you get to modern society and women like Amanda Beard** (an olympian swimmer) crop up. Again, I voice a question I asked before: If large breasts confer any kind of advantage, be it for sexual selection or natural selection, why aren't large breasts the standard?
Although, I guess I can't know that for sure. Somebody would have to run one helluva study to figure out the "average" breasts size among various populations of women. Would that average change across traditionally "racial" or culture boundaries? I just got a cool book which features Playboy centerfolds for 54 years, from 1953-2007, and intend to figure out the average myself, leaning on the assumption that Playboy is an accurate barometer of the "American Girl." What I'm finding difficult here is that measurements are not always known and are sometimes simply incorrect, so...yeah***. This is a story for another day, though.****
I don't really see a reason why large breasts can't be the result of both sexual selection and natural selection. It would be advantageous for men to find a naturally selected feature attractive, as it would more quickly disperse that trait among the gene pool. Look at the whole mess of display organs in animals. Dimetrodon has a giant dorsal sail that probably functioned as a thermoregulatory device but could have easily functioned for sexual signaling. Modern giraffes have long necks for both reaching high vegetation (and thus neatly avoiding competition) and battling with rivals. Triceratops had horns and frills that have been shown to have intraspecific display/battle functions, but you can damn well be sure that it's gonna defend itself against a marauding tyrannosaur with that same weaponry. A single structure can have multiple uses.
And another reason why small breasts might successfully "infiltrate" any sort of norm could be just the fact that our genes are spread extremely far, and a whole host of new phenotypes have popped up as a result. And thanks to the same mechanism, no matter what the phenotype, somebody will find it attractive (on a man or a woman), so it successfully remains in that population's gene pool. Anyway, go ahead and read the article, and post your thoughts below!
*And if large breasts are such an advantage during the dry season, why wouldn't other non-human primates develop them?
**Amanda Beard might be a poor choice. Atheletes tend to lose every inch of body fat, especially olympians. Ms. Beard might have had larger breasts before diving into the pool. She's just the only example I could come up with on a whim.
***Playboy likes to pretend the breasts don't get much larger than a DD. There are only four, maybe five examples of playmates with cited busts of DD, and I question whether these measurements are actually too conservative. I could list some names, but this is honestly a topic for an entirely separate post.
****Yes, my wife is aware of the book. She thinks I could publish such a study in a social sciences journal.